A conventional known exposure control circuit employing a silicon photodiode (hereinafter abbreviated to SPD) as a light receiving element is by way of example shown in FIG. 4, in which numeral 1 denotes an SPD forming a light receiving element, 2 an operational amplifier, 3 a diode forming a logarithmic compression element, 4 a bias power source, 5 a transistor forming a logarithmic expansion element, 6 an integrating condenser, 7 a switching transistor which is cut off in interlocked relationship with the exposure operation, 8 a comparator, and 9 a magnet which is engaged with a member closing shutter blades to block operation of the member. The diode 3 is formed by short-circuiting between a collector and a base of a transistor.
In the above circuit, when the SPD 1 is exposed to light from the subject to be photographed, the SPD 1 produces a photocurrent corresponding to the brightness of a subject to be photographed and the photocurrent flows through the diode 3. A signal having a level obtained by adding a logarithmically compressed level of the photocurrent by the diode 3 to a bias voltage determined by the bias power source 4 appears at the output of the operational amplifier 2. The output of the operational amplifier 2 is applied to a base of the transistor 5 so that a logarithmically expanded currrent thereof flows through the transistor 5.
In an initial state, the transistor 7 is conductive to short-circuit the condenser 6 so that the condenser 6 is not charged. However, when the transistor 7 is cut off in interlocked relationship with the exposure operation, the condenser 6 is charged by the expanded current flowing through the transistor 5 to reduce its charged level, that is, the level at the junction between the condenser 6 and the transistor 5. The charged level of the condenser 6 is applied to an inverted input of the comparator 8. Since a predetermined reference level Vref is applied to a non-inverted input of the comparator 8, when the charged level to the condenser 6 is reduced to the reference level Vref, the output of the comparator 8 is inverted to a high level to deenergize the magnet 9 so that the shutter blades are closed.
Since the photocurrent flowing through the SPD 1 is converted to the charging current of the condenser 6 through the logarithmic compression by the diode 3 and the logarithmic expansion of the transistor 5, the charged time of the condenser 6 is reduced to half for each rise of one step on the Bv value of the brightness of the subject in the case where the .gamma. value of the SPD 1 is 1. The .gamma. value is defined by a ratio of an amount of variation of the photocurrent and an amount of variation of the brightness of the subject and the .gamma. value is 1 in the case where the photocurrent doubles each time the brightness is increased by one step on the Bv value.
Accordingly, the exposure control circuit constructed as shown in FIG. 4 and employing the SPD 1 having the .gamma. value of 1 as described above is suitable for a so-called diaphragm preference type automatic exposure control using, for example, a focal-plane shutter.
FIG. 5 shows a conventional known example illustrating a simplest structure of the programming shutter provided with shutter blades having the function of the diaphragm which can be applied to the present invention as it is. In FIG. 5, numeral 61 denotes a base plate for the shutter, 62 and 63 shutter blades, respectively, having the function of the diaphragm, and 64 a lever for opening and closing the shutter blades.
The shutter blades 62 and 63 are swingably supported on the surface of the base plate 61 by an axis 61a mounted on the base plate 61. The lever 64 is swingably supported on the rear surface of the base plate 61 by an axis 61b mounted on the base plate 61.
An elongated hole 62a is formed in the shutter blade 62 and a boss 64b formed at the center of the lever 64 is engaged with the elongated hole 62a through an elongated hole 61c formed on the base plate 61.
Similarly, an elongated hole 63a is also formed in the shutter blade 63 and a boss 64b formed at the center of the lever 64 is engaged with the hole 63a through an elongated hole 61d of the base plate 61.
The lever 64 is urged to rotate counter-clockwise by a spring 65, while in the shutter set condition (illustrated in FIG. 5), the lever 64 is engaged with an engagement member not shown to prevent its counter-clockwise rotation.
When the engagement of the lever 4 is released in interlocked relationship with a stroke of shutter button not shown, the lever 64 is rotated counter-clockwise by the spring 5 so that the counter-clockwise rotation of the lever 64 rotates the shutter blade 62 counter-clockwise about the axis 61a through the boss 64a and at the same time rotates the shutter blade 63 clockwise about the axis 61a through the boss 64b.
Accordingly, an aperture 61e is opened by the shutter blades 62 and 63 and the diameter of the aperture is gradually increased. When the exposure is terminated, the lever 64 is returned to its initial position by the opposite movement to the opening of the aperture to return the shutter blade 62 and 63 to the initial position thereof.
FIG. 6 is an opening characteristic diagram of the programming shutter having the structure as shown in FIG. 5, and in FIG. 6 the axis of abscissa indicates the exposure time t and the axis of ordinate indicates the opening area A of the aperture.
There are various opening characteristics for such a shutter depending on shapes of the shutter blades 62 and 63 and structures of drive mechanisms for the shutter blades. For example, when the aperture is adapted to be opened proportionally to the exposure time t, the opening area A is proportional to the square of the exposure time in the so-called triangular opening area before the aperture is fully opened and the exposure amount S is proportional to an integrated value of the opening area A with the exposure time t.
In the programming shutter, since the exposure amount S is proportional to the exposure time t in the fully-opened area after the aperture has been fully opened, the SPD having the .gamma. value of 1 can be used as a light receiving element without any problem. However, since the light receiving amount per unit time is varied momentarily in the triangular opening area before the aperture is fully opened, the SPD 1 having the .gamma. value of 1 can not be used as it is.
Various countermeasures thereof have been known heretofore and are basically divided into two manners. The first manner is to employ an auxiliary diaphragm blade so that the exposure area of the SPD 1 is followed to the opening characteritic of the shutter blades 62 and 63.
More particularly, if the light receiving area of the SPD 1 is adapted to be increased in interlocked relationship with the opening operation of the shutter blades 62 and 63, the light receiving amount of the SPD 1 is increased in accordance with the increase of the light receiving amount of the film surface. Accordingly, even if the SPD having the .gamma. value of 1 is used as the light receiving element, a proper exposure control can be attained even in the triangular opening area. However, in this manner, the shape of the shutter blades is larger due to necessity of an auxiliary diaphragm blade and the design of the shape of the shutter blades and the disposition of the SPD 1 are greatly limited.
The second manner is to properly adjust a level of the bias power source 4 with the lapse of the exposure time in the triangular opening area so that the charged current to the condenser 6 is corrected with the lapse of the exposure time.
However, in this manner, it is very delicate and difficult to adjust the level of the bias power source 4. Further, when the level of the bias power source 4 is constant regardless of temperature, the charged current to the condenser 6 contains parameter proportional to the absolute temperature and accordingly it is difficult to obtain a satisfactory temperature characteristic as a circuit assembled in a camera which is used in the normal temperature.
An element of determining the charged current of the condenser 6 involves a film speed in addition to the brightness of the subject. As well known, it is necessary to introduce the element associated with the film speed into the circuit of FIG. 4.
In the case where the film speed is introduced into the circuit of FIG. 4, the level of the bias power source 4 is normally adjusted in interlocked relationship with a setting mechanism of the film speed.
In the circuit of FIG. 4, each time the brightness of the subject is varied one step on the Bv value, the output level of the operational amplifier 2 is varied about 18 mV (hereinafter abbreviated merely to 18 mV). Since the Sv value indicating the film speed and the Bv value indicating the brightness of the subject have the same weight for the exposure control, it is required to configure the bias power source 4 so that the level of the bias power source 4 is varied by 18 mV each time the film speed is varied one step on the Sv value.
Further, since the film speed is normally established for each one-third step on the Sv value and requires a very wide range, when the film speed is introduced into the circuit by the adjustment of the level of the bias power source 4, it is required to adjust the level of the bias power source 4 with high accuracy over the wide range. However, so long as the film speed is introduced by the level adjustment of the bias power source 4 even if the level is adjusted with high accuracy, the charged current of the condenser 6 contains the parameter proportional to the absolute temperature and it is difficult to obtain the satisfactory temperature characteristic as the circuit assembled in the camera which is used in the normal temperature.